467 research outputs found
A new experimental procedure for characterizing quantum effects in small magnetic particle systems
A new experimental procedure is discussed, which aims at separating thermal
from quantum behavior independently of the energy barrier distribution in small
particle systems.
Magnetization relaxation data measured between 60 mK and 5 K on a sample of
nanoparticles is presented. The comparison between experimental data and
numerical calculations shows a clear departure from thermal dynamics for our
sample, which was not obvious without using the new procedure presented here.Comment: LaTeX source, 6 pages, 5 PostScript figure
Aging, rejuvenation and memory phenomena in spin glasses
In this paper, we review several important features of the out-of-equilibrium
dynamics of spin glasses. Starting with the simplest experiments, we discuss
the scaling laws used to describe the isothermal aging observed in spin glasses
after a quench down to the low temperature phase. We report in particular new
results on the sub-aging behaviour of spin glasses. We then discuss the
rejuvenation and memory effects observed when a spin glass is submitted to
temperature variations during aging, from the point of view of both energy
landscape pictures and of real space pictures. We highlight the fact that both
approaches point out the necessity of hierarchical processes involved in aging.
Finally, we report an investigation of the effect of small temperature
variations on aging in spin glass samples with various anisotropies which
indicates that this hierarchy depends on the spin anisotropy.Comment: submitted for the Proceedings of Stat Phys 22, Bangalore (India
Extraction of the Spin Glass Correlation Length
The peak of the spin glass relaxation rate, S(t)=d{-M_{TRM}(t,t_w)}/H/{d ln
t}, is directly related to the typical value of the free energy barrier which
can be explored over experimental time scales. A change in magnetic field H
generates an energy E_z={N_s}{X_fc}{H^2} by which the barrier heights are
reduced, where X_{fc} is the field cooled susceptibility per spin, and N_s is
the number of correlated spins. The shift of the peak of S(t) gives E_z,
generating the correlation length, Ksi(t,T), for Cu:Mn 6at.% and
CdCr_{1.7}In_{0.3}S_4. Fits to power law dynamics, Ksi(t,T)\propto
{t}^{\alpha(T)} and activated dynamics Ksi(t,T) \propto {ln t}^{1/psi} compare
well with simulation fits, but possess too small a prefactor for activated
dynamics.Comment: 4 pages, 4 figures. Department of Physics, University of California,
Riverside, California, and Service de Physique de l'Etat Condense, CEA
Saclay, Gif sur Yvette, France. To appear in Phys. Rev. Lett. January 4, 199
Spin Glasses: Model systems for non-equilibrium dynamics
Spin glasses are frustrated magnetic systems due to a random distribution of
ferro- and antiferromagnetic interactions. An experimental three dimensional
(3d) spin glass exhibits a second order phase transition to a low temperature
spin glass phase regardless of the spin dimensionality. In addition, the low
temperature phase of Ising and Heisenberg spin glasses exhibits similar
non-equilibrium dynamics and an infinitely slow approach towards a
thermodynamic equilibrium state. There are however significant differences in
the detailed character of the dynamics as to memory and rejuvenation phenomena
and the influence of critical dynamics on the behaviour. In this article, some
aspects of the non-equilibrium dynamics of an Ising and a Heisenberg spin glass
are briefly reviewed and some comparisons are made to other glassy systems that
exhibit magnetic non-equilibrium dynamics.Comment: To appear in J. Phys.: Condens. Matter, Proceedings from HFM2003,
Grenobl
Numerical Study on Aging Dynamics in the 3D Ising Spin-Glass Model. II. Quasi-Equilibrium Regime of Spin Auto-Correlation Function
Using Monte Carlo simulations, we have studied isothermal aging of
three-dimensional Ising spin-glass model focusing on quasi-equilibrium behavior
of the spin auto-correlation function. Weak violation of the time translational
invariance in the quasi-equilibrium regime is analyzed in terms of {\it
effective stiffness} for droplet excitations in the presence of domain walls.
Within the range of computational time window, we have confirmed that the
effective stiffness follows the expected scaling behavior with respect to the
characteristic length scales associated with droplet excitations and domain
walls, whose growth law has been extracted from our simulated data. Implication
of the results are discussed in relation to experimental works on ac
susceptibilities.Comment: 18 pages, 6 figure
A Serravallian (Middle Miocene) shark fauna from Southeastern Spain and its palaeoenvironment significance
The study of a new Serravallian (Middle Miocene) locality from the Southeastern Spain has yielded a shark assemblage characterized by microremains of at least seven taxa (Deania calcea, ¿Isistius triangulus, ¿Squaliolus cf. S. schaubi, ¿Paraetmopterus sp., Pristiophorus sp., Scyliorhinus sp. and a cf. Squaliformes indet) of three different orders (Squaliformes, Pristiophoriformes and Carcharhiniformes). In addition, associated macroremains have also been found, including teeth of ¿Cosmopolitodus hastalis, Isurus sp., Hemipristis serra, Odontaspis sp., Carcharhinus spp. and ¿Otodus (Megaselachus) megalodon. The assemblage contains taxa with disparate environmental preferences including not only neritic and epipelagic sharks but also an important number of meso and bathypelagic representatives. The migration of deep water taxa to shallower waters through submarine canyons/coastal upwelling is proposed as the most plausible cause for explaining the origin of such assemblage. Interestingly, the composition of the deep-water taxa here reported contrast with the chondrichthyans assemblages from the Pliocene and extant Mediterranean communities. This entails a complex biogeographic history, where the Messinian salinity crisis strongly affected the posterior evolution of the Mediterranean ecosystems but some other factors, such us the existence of anoxic events during the Quaternary, could have also played an important role
A Crash Course on Aging
In these lecture notes I describe some of the main theoretical ideas emerged
to explain the aging dynamics. This is meant to be a very short introduction to
aging dynamics and no previous knowledge is assumed. I will go through simple
examples that allow one to grasp the main results and predictions.Comment: Lecture Notes (22 pages) given at "Unifying Concepts in Glass Physics
III", Bangalore (2004); to be published in JSTA
Experimental constraints on the -nucleus real potential
In a search for mesic states, the production of -mesons in
coincidence with forward going protons has been studied in photon induced
reactions on C for incident photon energies of 1250 - 3100 MeV. The
pairs from decays of bound or quasi-free -mesons have
been measured with the CBELSA/TAPS detector system in coincidence with protons
registered in the MiniTAPS forward array. Structures in the total energy
distribution of the pairs, which would indicate the population
and decay of bound B states, are not observed. The
cross section of 0.3 nb/MeV/sr observed in the bound state energy regime
between -100 and 0 MeV may be accounted for by yield leaking into the bound
state regime because of the large in-medium width of the -meson. A
comparison of the measured total energy distribution with calculations suggests
the real part of the B potential to be small and only
weakly attractive with 35(stat) 20(syst) MeV
in contrast to some theoretical predictions of attractive potentials with a
depth of 100 - 150 MeV.Comment: 13 pages, 8 figure
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